Answer:
The movement of the earth.
Step-by-step explanation:
"The earth's spin axis is tilted. This is what causes the seasons. When the earth's axis points towards the sun, it is summer for that hemisphere. When the earth's axis points away, winter can be expected."
Because the earth is constantly moving/spinning *although slowly*. Depending on where the earth is tilted or what side the sun is pointing to will decided the seasons; as mentioned if the earth's axis is pointed away winter can be expected in the said hemisphere.
although Hemshpheres are a whole other deal that shouldn't be needed for this question.